1. Field of the Invention
The present invention relates to an image information reading apparatus which corrects a shading distortion of a light source based on data stored in memory means, when image information is read.
2. Description of the Prior Art
In an apparatus such as a copying machine or a facsimile machine which illuminates an original document image by means of a light source and processes an image based on reflected light, a shading distortion correction is essential. In a plane scan type facsimile machine which uses a linear light source such as a fluorescent lamp, a light intensity is insufficient at an edge of the original document and a shading distortion occurs. In the past, an image information read apparatus having a shading distortion correction device as shown in FIG. 1 is used. In FIG. 1, a standard white plate 13 having a uniform reflection coefficient which is illuminated by a light source L through a slit Ps formed in an original document platform P is focused onto an photoelectric conversion means 12 (e.g. CCD) through a mirror M and an optical system 11 such as a lens. The photoelectric conversion means 12 sequentially reads out information under the control of a read controller C which controls device timing, and the read information is amplified by an amplifier A, converted to a digital signal by an A/D converter 3 and the digital signal is sequentially stored in a memory 4 in accordance with an address signal supplied from the read controller C.
If a shading distortion is included, the output from the amplifier A has a waveform as shown in FIG. 2A. It is quantized into a waveform as shown in FIG. 2B and the respective bits are stored at the corresponding addresses of the memory 4. This data represents a shading distortion data (light intensity distribution data).
FIG. 3 shows a block diagram of the circuit surrounded by a broken line l in FIG. 1. In FIG. 1, numeral 1 denotes an initial value setting circuit which receives the output of the photoelectric converter such as CCD which receives a light reflected from the standard white plate illuminated by the light source. The initial value signal is supplied to a peak detector 2 which detects a peak value 12a shown in FIG. 2A, and the signal is converted to a digital signal by the A/D converter 3 with reference to the peak value. The digital data is then stored in the memory 4.
In a read operation, the data stored in the memory 4 is sequentially read out in accordance with a read scan position, converted to analog data by the D/A converter 6, the output of which is supplied to a comparator 8 to correct a slicing level.
A video signal from a video signal circuit 7 derived by the read scan is supplied to the comparator 8 and a binary output signal 9 is produced by the corrected slicing level.
In this system, the shading distortion correction is essentially correctly made if the initial value is correctly set. However, the correct initial value may not be set depending on conditions such as operating method.
For example, in a facsimile machine, when an operator wants to transmit an image from an intermediate point of an original document, the original document is inserted up to the read position. Since the standard white plate 13 is usually arranged near the read position in the facsimile machine as shown in FIG. 1, the inserted original document 10 is in front of the standard white plate 13. Thus, the original document image is read when the initial value is to be set and the standard white plate 13 is hidden by the original document 10. As a result, a correct initial value is not set and a correct binarization is not attained.